Conventional Selective Compliant Articulated/Assembly Robot Arms (SCARA) in the market are driven through transmission systems, such as gear trains, belts-and-pulleys, chains and linkages, which are used to transmit power from the drive motors to the load. The motors are typically located at the base of the robot structure, and the upper arms are moved by the transmission systems linking the drive motor to the arms.
Although transmission systems provide increased drive torques, they have inherent imperfections, such as friction and wear in the transmission elements, which result in degradation of performance over time. Transmission systems are also subject to backlash and hysteresis, which introduces inaccuracy in positioning the end-effector. Further, the compliance chain in transmission from the motor to the end-effector results in reduced controllability of each axis of the robot and thus degrades performance in terms of lower operation bandwidth. To overcome these limitations, additional complicated and bulky mechanisms have to be employed. In addition, careful adjustment and regular maintenance of the transmission systems are necessary. These measures inevitably increase costs of constructing and maintaining a conventional robot.
Another limitation of conventional robots is the indirect measurement of the angular position of the robot arms. Typically, angular position of the arm is deduced from the rotation of the motor which indirectly drives the arm through a transmission system. Indirect measurement of the arm position leads to significant errors in ascertaining the position of the end effector. In high precision applications such as semiconductor fabrication, such positioning errors are unacceptable.
In view of the foregoing limitations, it is desirable to provide a robotic manipulator configuration which eliminates the imperfections of transmission systems and also improves accuracy of positional measurement and feedback.